I don't think you've explained what the actuator is used for. Is someone going to be sitting in this and using it to raise/lower the chair? If such is the case then you just need a DPDT switch to move it up and down.

What the actuator is going to do isn't really important, but in this case it will let a shovelblade up and down

I see a couple of possibilities:

1) If the dozer is always in view of the operator, then the Arduino isn't really needed: The actuator should have its own built-in limit switches (check on this first, though), and you only need to switch the polarity of the motor (in whatever manner the actuator needs) to change the direction of travel. The operator can see where the blade is, etc. Then, all you need is a way to switch the direction of the motor with the R/C controls. There do exist boards for the R/C crowd that allow you to switch relays or such using your transmitter (they are commonly used for auxiliary items in R/C). Or, you could use a small R/C servo to switch a couple of regular switches mechanically.

2) If you really wanted to use the Arduino - then you could use it to decipher the servo signal stream output by the R/C receiver for the channel for blade control, and convert that PPM signal into something to tell the Arduino to output on a couple of digital pins LOW-LOW (no movement), HIGH-LOW (movement in one direction) and LOW-HIGH (movement in the other). This is kinda what your diagram seems to indicate (so you seem to understand that much). This is essentially what the first option above does, though (though it may not necessarily use a microcontroller to do it - it is possible to do the whole thing without needing a microcontroller).

For bonus points, if the actuator has a position output sensor (potentiometer or pulse counter), you could read that to determine a stopping point based on its output (in the case of a pulse sensor, you would have to set things up to allow the position of the actuator to "hit the stops", so you could hit a stop, then reverse direction to count the number of pulses to the other stop as a calibration procedure on power-up, so the Arduino can know, based on pulse counts, where the blade is positioned).

Now - if the dozer is out-of-sight, remotely operated with a camera/light system on-board (?) - having the Arduino be able to read the position could be helpful in that you could send that telemetry back to the control station, and use it for some kind of indicator to show the blade height for the machine (since is may not be able to be seen directly).

I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

With the Arduino you could make the robot more autonomous. You could add some sensors that would detect that it was against the wall of the tunnel and adjust track speed to keep it parallel. Or that could detect some distance from the wall and use it to guide the robot straight and parallel but ofset from the wall for each pas down the duct. You could add sensors to detect where ridge in the pile of sand is and adjust the robots position to keep too much sand from dumping off the side of the blade that is in the area that has already been cleaned. If you did something like that then your RC link would be more of a guidance and override rather than CONTROL.

One thing you might consider using the Arduino for is better control of the drive motors. I have a set up very much like this (differential drive motors controlled by R/C transmitter) that works, but I have a hell of a time keeping the drive motors balanced enough to drive in a straight line. I'm using old wheelchair motors connected to a Sabertooth 2x25 and controlled by a cheap R/C transmitter (FS-CT6A) via the R/C receiver connected directly to the Sabertooth. Now I'm working on a way to use the Arduino to help balance the two motors and make driving in a straight line a little less impossible. My rough plan is to reconfigure the R/C controller so that the Right stick controls forward and reverse for both motors and the Left stick will control steering. The R/C signal will be passed to the Arduino by the R/C receiver. The Arduino won't do much for forward and reverse except apply those signals to both motors at the same PWM. When you want to turn, however the Arduino programming will apply a duty cycle change to both motors to either "speed up" or "slow down" either the right or left motor - and in theory effecting a very controlled turn. The two problems that I'm working out now are how to accomplish a zero radius turn without forward or reverse motion and how to build in a fail safe. The Sabertooth will stop all motors the instance that it loses the R/C signal - so if it's directly connected to the R/C receiver and it starts going crazy all you need to do is turn off the R/C transmitter. The problem is that adding the Arduino between the receiver and Sabertooth might cause problems with this safety feature... Hopefully that will help some - by the way awesome build!